Archived posting to the Leica Users Group, 1997/03/17
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Peter Jon White wrote: >Light reflected off of a glass surface is polarized. The surface >reflects more light of one polarity than the other. It acts in the >same fashion as a linear filter. So any internal meter than measures >light reflected off a mirror is reading polarized light, whether or >not you have a polarizing filter on the lens. This is only partly true. Light reflected off of a glass surface _is_ polarized, as stated. However, light reflected off of a polished metal surface, which applies to all SLR mirrors that I know of, is _not_ polarized. That's why cameras with the metering sensors in the prism do not need circular polarizers, while cameras with light sensors in the bottom of the mirror box do need circular polarizers. The latter measure light that is transmitted through a semi-silvered mirror, which does polarize light, and then that light is reflected off another mirror, which may also polarize before it gets to the light sensor. Many cameras use a combination of metering systems. The Nikon F4, for example, uses prism mounted sensors for matrix and center-weighted metering, and a mirror-box bottom sensor for spot metering, and a mirror-box bottom array for autofocus. So if you want to use a polarizer, but only have a linear type for a certain lens, you could manually focus and use matrix or center-weighted metering, and not have any problem due to the polarizer. * /|\ Henning J. Wulff /###\ Architecture |[ ]| henningw@portal.ca